Immunity is one of self-protecting biological defense mechanisms against invading foreign organisms such as bacteria and viruses and includes innate immunity associated with phagocytosis of leukocytes etc. and acquired immunity against a specific antigen/pathogen. A vaccine is a medicament for safely inducing the acquired immunity against a pathogen and recently has been used not only for protection but also for treatment.
A vaccine includes a live vaccine, an inactivated vaccine, a component vaccine, and the like. A live vaccine is highly immunogenic and has a prolonged immunological effect but also has a risk that pathogenicity remains or is reverted. An inactivated vaccine, prepared by treating viruses with formalin etc. to remove the pathogenicity, is safer than a live vaccine but has a defect that the resulting immunity is not prolonged.
A component vaccine comprises as a main component an antigenic protein, prepared by extracting and purifying a protein having a vaccinal effect from a pathogen or artificially prepared by genetic engineering techniques or chemical procedures, and is highly safe free from contamination of unwanted proteins. Recently, a peptide vaccine as one of component vaccines has been studied profoundly. A peptide refers to a molecule that consists of amino acids bound to each other through peptide linkage. Generally, a peptide with 10 or less amino acid residues is called an oligopeptide, a peptide with less than 100 amino acid residues is called a polypeptide (both an oligopeptide and a polypeptide are herein referred to as a “peptide”), and one with 100 or more amino acid residues is called a protein.
Recently, amino acid sequences of various proteins and viral antigenic proteins have been determined. A peptide artificially synthesized based on such an amino acid sequence is used as a vaccine, which is referred to as a peptide vaccine. The advantage of using a peptide as a vaccine is that a highly pure antigen can be obtained artificially without handling pathogenic microorganisms. On the other hand, it is difficult to obtain a sufficient immune effect since a peptide has a small molecular weight and hence is not recognized as a foreign substance in the living body on immunization. Hence, a peptide is combined with a large protein called a carrier protein so as to be recognized as a foreign substance and/or is administered together with an adjuvant (immunomodulator) in order to enhance immune effect. However, with these treatments, it is possible that an antibody against a carrier protein may be produced or an adverse side effect of an adjuvant may unexpectedly be induced. Besides, while an adjuvant is proved to be effective on research level, it is only an aluminum hydroxide gel that is permitted in Japan for usage in human.
On the other hand, a peptide vaccine has been attempted for use for preventing and/or treating diseases such as Alzheimer disease. Alzheimer disease is one of dementia disorders and is associated with declined cognitive function and change in personality as principal symptoms. The increasing number of patients along with rapid increase of aging population has become a social issue. Alzheimer disease's pathological indications include three features of atrophy and/or fall-off of neurons, formation of senile plaques due to aggregation and/or deposition of Aβ and neurofibrillary changes due to abnormal tau proteins. Onset of Alzheimer disease is initiated by deposition of Aβ peptides (senile plaque formation) followed by denaturing and fall-of neurons with increase in Aβ deposition. The deposition of Aβ peptides then trigger deposition of tau proteins followed by neurofibrillary changes. Aβ peptide is derived from an amyloid peptide precursor (APP) existing in the brain and the body. In normal process, APP is cleaved by α-secretase in the middle and then by γ-secretase in the C-terminal to generate a P3 peptide which is subsequently degraded completely. In the case of Aβ peptide deposition, APP is cleaved by β-secretase and then by γ-secretase in the C-terminal to generate Aβ peptides consisting of 40 or 42 amino acid resides (Aβ40, Aβ42). Among these, Aβ42, easily aggregated and deposited, is extracellularly secreted to be insolubilized, and aggregated and deposited to form senile plaques. Increase in production and accumulation of Aβ42 peptides would affect a synapse. Further, microglial cells and astrocytes are gathered around the aggregated Aβ peptides. It is thought that damages in the synapse and the neurite further progress to lead to degeneration and cell death of neurons, resulting in dementia.
Nowadays, targeting Aβ peptides, a method of treatment is considered for decreasing Aβ peptides, including, for instance, inhibition of the action of secretases which produce Aβ peptides, use of an Aβ degrading enzyme which may degrade the produced Aβ peptides, use of a vaccine or an antibody for removing those extracellularly excreted and those aggregated, and the like.
Approach of treating Alzheimer disease with a vaccine was first reported by Schenk et al. (Non-patent reference 1), which comprises administering Aβ42 peptides together with an adjuvant by intramuscular injection to thereby produce an antibody against Aβ to remove the accumulated Aβ peptides. A clinical trial for the vaccine was performed by administering intramuscularly a medicament comprising the Aβ42 peptide together with a purified saponin as an adjuvant. As a result, it was shown that an antibody specific to Aβ peptide was produced in Alzheimer disease patients by administration of the vaccine and that the production of the antibody specific to Aβ peptide could retard the development of cognitive disturbance in Alzheimer disease patients (Non-patent reference 2) and it was proved that senile plaques were disappeared (Non-patent reference 3). However, since serious meningoencephalitis was observed in some subjects, the clinical trial was discontinued. It is supposed that one of causes of the adverse side effect is the adjuvant contained in the vaccine. Accordingly, for a peptide vaccine, development of a formulation that is efficient and safe is strongly desired.    Non-patent reference 1: Schenk D, Barbour R, Dunn W, Gordon G, Grajeda H, Guido T, et al., Immunization with amyloid-beta attenuates Alzheimer disease -disease-like pathology in the PDAPP mouse. Nature 1999; 400: p. 173-177    Non-patent reference 2: Fox N C, Black R S, Gilman S, Rossor M N, Griffith S G, Jenkins L, et al. Effects of A beta immunization (AN1792) on MRI measures of cerebral volume in Alzheimer disease. Neurology 2005; 64: p. 1563-1572    Non-patent reference 3: Nicoll J A, Wilkinson D, Holmes C, Steart P, Markham H, Weller R O. Neuropathology of human Alzheimer disease after immunization with amyloid-beta peptide: a case report. Nat Med 2003; 9: p. 448-452